Lubricating oils are, for the most part, based on petroleum fractions that boil above about 232 degree C (450 degrees F). The molecular weight of the hydrocarbon constituents is high and these constituents display almost all conceivable structures and structure types depending in large part on the type of crude oil from which they were prepared.
The rationale in lubricant refining is that a suitable crude oil, as shown by experience or by assay, can be refined into lubricant base stock having a predetermined set of properties such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The refining process employed to isolate the lubricant base stock currently consists of a set of subtractive unit operations which remove the unwanted components. These unit operations include distillation, solvent refining, hydroprocessing, and dewaxing, each of which is basically a separation process.
A lubricant base stock (i.e. from a refined crude oil) may be used as a lubricant component, or it may be blended with another lubricant base stock having somewhat different properties. A particular base stock, prior to use as a lubricant, is conventionally compounded with one or more additives such as antioxidants, extreme pressure additives, and viscosity index (V.I.) improvers. As used herein, the term "stock," regardless of whether the term is further qualified, refers only to a hydrocarbon oil without additives. The term "solvent-refined stock" or "raffinate" refers to an oil that has been solvent extracted, for example with furfural. The term "dewaxed stock" refers to an oil which has been treated by any method to remove or otherwise convert the wax contained therein and thereby reduce its pour point. The term "waxy" as used herein refers to an oil of sufficient wax content to result in a pour point greater than 4 degrees C. (+25 degrees F.). The term "base stock" refers to an oil refined to a point suitable for some particular end use, such as for preparing an automotive oil, marine oil, hydraulic oil, etc.
The current practice for the preparation of high grade lubricating oil base stocks is illustrated in FIG. 1. The overall process is designated as 10. The first step is to vacuum distill an atmospheric tower residuum from an appropriate crude oil (step 100). This step provides one or more raw stocks within the boiling range of about 700 degrees F to about 1000 degrees F designated as light 15 (from about 700 to about 810 degrees F), medium 20 (from about 810 to about 890 degrees F), and heavy 30 neutrals (from about 890 to about 1000 degrees F), and a vacuum residuum 40. Each stock is characterized by a different viscosity range, e.g., light neutral stocks have the lowest viscosity range (from about 3.5 to about 5.5 Kv at 100 degrees C) and heavy neutral stocks have the highest viscosity range (from about 10.0 to about 15.3 Kv at 100 degrees C). After preparation, each raw stock is extracted with a solvent, e.g., furfural, phenol or chlorex, which is selective for aromatic hydrocarbons, and which removes undesirable components (step 140). The vacuum residuum 140 usually requires an additional step to remove asphaltic material prior to solvent extraction (step 120). The raffinate from the solvent refining is generally very waxy and typically requires a dewaxing operation (step 160). Dewaxing raffinates is generally carried out by solvent dewaxing or catalytic dewaxing under conditions which produce a predetermined or target pour point for the base stock.
The art of lube base stock production may further require an additional step of hydroprocessing the dewaxed stock (step 180). Hydroprocessing is the addition of hydrogen to the stock for the purpose of removing certain impurities that may be detrimental to lubricant performance, depending upon the final lubricant product specifications. Hydroprocessing is especially useful for removing sulfur as the hydrogen combines with sulfur to form hydrogen sulfide. Either the dewaxing step (step 160) or the hydroprocessing step (step 180), or both, can yield a stock of sufficient quality to be used as a base stock for final lubricant formulation.
Suitable materials, or additives, are then added to the base stock to augment the base stock's lubricating properties and to meet the required specifications, such as viscosity index (VI) (shown in FIG. 6).
Historically, lubricant manufacturing has been inflexible. Base stock produced from new crude sources is considered to be a "new" base stock and must be approved for use as a lubricant. The lube base stock approval system requires that a base stock produced from a new crude undergoes a costly and time-consuming approval process before it can be used as a lubricant oil. Full approval for samples produced from a new crude or base stock requires extensive bench and engine testing which generally takes 5 to 10 months to complete. Changes in process conditions also require re-approval with additional bench and engine testing. The high cost of bench and engine testing discourages optimization of process conditions on all but the most frequently run crudes. In addition, the need to streamline this system has become increasingly important as a result of the rapid changes in crude mixes from at least some geographic locations, quality concerns over purchased base stocks, and the drive for economic benefits from increased lube flexibility. Therefore, there is a need in the art for increased flexibility in lubricant production and to streamline the approval system for lube crudes, base stocks, and blends thereof.